The present invention relates to apparatus for receiving and processing a radio frequency signal and to a method of receiving and processing a radio frequency signal, and, in particular, to a radio receiver for use in a portable communications device, in which the radio signal to be received is directly down converted to In-phase (I) and Quadrature-phase (Q) signals centred around an Intermediate Frequency (IF) which is of the same order of magnitude as the bandwidth of the signal to be received.
Most conventional radio receivers for use in portable communication devices such as cellular telephones, are of the super-heterodyne type in which a radio signal to be received is first down-converted to an IF which is still in the Radio Frequency (RF) range and then further down-converted to a base-band signal (having both I and Q components) from which the information contained in the signal may be recovered. Such a receiver is robust. However, direct conversion receivers and, more recently, Very Low IF (VLIF) receivers have been proposed in order to reduce costs by eliminating both a relatively high performance, and therefore expensive, Surface Acoustic Wave (SAW) band-pass filter (for allowing the wanted IF signal to pass while blocking all unwanted IF signals in neighboring channels) and one of the two RF Local Oscillators (LO) required in super-heterodyne receivers.
Direct conversion receivers immediately down convert the received radio signal to a base-band signal thus completely eliminating the IF stage. However, such receivers suffer from the formation of a very large unwanted Direct Current (DC) noise component. This DC noise component is formed largely by leakage from the Local Oscillator being received at the receiver aerial together with the wanted signal, and also by offsets of the amplifiers and mixers in the receivers. This unwanted DC noise component must clearly be removed if the information contained in the signal is to be successfully recovered. However, because the noise to be removed is located at DC, a significant amount of time is required for a suitable DC notch filter (ie a high pass filter having a corner very close to DC with a very steep fall-off characteristic) to adapt to the correct amount of DC to remove. This time may be referred to as the DC offset adept period. The existence of the DC offset period requires that the receiver effectively be switched on some time prior to receiving the wanted signal. Furthermore, since the unwanted DC noise component is located in the middle of the wanted signal, a significant amount of useful information contained within the wanted signal will also be lost when the DC noise component is filtered out.
In order to overcome this problem, a VLIF receiver has been proposed in which the received signal is first down-converted to be centred about an IF which is equal to approximately half the channel spacing (ie approximately half the band-width of the wanted signal), and then it is down-converted again to base-band. In this way, the DC component, which is still formed when the first down conversion takes place, is located (in frequency) at the very edge of the wanted signal. From here, the unwanted DC component may be filtered out without losing so much information contained in the wanted signal. Furthermore, if one removes this unwanted noise component after down conversion from IF to base-band, the noise component will now be located away from DC and thus a suitable notch filter can remove this component without requiring the receiver to be turned on for the significant DC offset adapt period.
However, it has been surprisingly discovered that, if one attempts to use a simple real (ie non-complex) filter which not only removes the unwanted noise component, but also filters out a corresponding portion of the signal having a frequency equal in magnitude to that of the shifted DC noise component but opposite sign, a significant amount of the information contained within the wanted signal is also lost by such filtering.
In accordance with a first aspect of the present invention, there is provided apparatus for receiving and processing radio frequency signals, comprising a radio frequency to intermediate frequency down-conversion stage for receiving a radio frequency signal and outputting an intermediate frequency signal; an intermediate frequency to baseband down-conversion stage for receiving the intermediate frequency signal and outputting a base-band signal; and a complex notch-filter for receiving the base-band signal and outputting a notch-filtered base-band signal, wherein the complex notch-filter substantially filters out a small portion of the base-band signal centred about a first, non-zero, frequency whilst substantially passing a corresponding portion of the base-band signal centred about a second frequency having the same magnitude but opposite sign to the first frequency.
The term base-band signal, will be well understood by a person skilled in the art as referring to the wanted signal centred about DC and having I and Q component signals which together represent the wanted signal as a complex signal, having both positive and negative frequency components.
By providing a complex notch filter, it is possible to remove the unwanted DC noise component located at one edge of the wanted signal without removing signal information from a corresponding portion of the wanted signal at its opposite edge. In this way, the filter does not require a long DC offset adapt period before it can accurately filter away the unwanted noise, as would be the case in a direct conversion receiver. On the other hand, it has been found that using a non complex notch filter is undesirable because of the useful signal information which is lost from the other edge of the wanted signal.
Preferably, the complex notch filter is programmable to enable the frequency of the small portion filtered out of the base-band signal to be altered as desired. This enables the apparatus for receiving and processing radio frequency signals according to the present invention to be easily modified to accommodate different standards (e.g. GSM, US TDMA, etc.).
Preferably, the complex notch filter has an asymmetrical response around the notch. By asymmetrical response, it is meant that the response of the filter is sharper (i.e. that it will remove less of the signal removed from the notch) on one side of the notch compared to the other. This is advantageous in the present invention since the wanted signal occurs substantially only on one side of the notch so that any removal of signal on the other side of the notch will not adversely affect the reception of wanted information contained within the wanted signal. Note that the ability to use an asymmetrical notch filter, requires that the notch filter be located not at DC (since this would then just be a simple high pass filter which cannot be made complex). For this reason, it is particularly advantageous to place the complex notch filter after the complex balanced multiplier or IF to base-band down-conversion stage.
Preferably, the apparatus includes an analogue to digital converter (ADC) which is arranged to convert one of the Radio Frequency (RF), Intermediate Frequency (IF) or base-band signals from an analogue into a digital signal. Ideally, the ADC is arranged to convert the IF signal from analogue into digital. Clearly, if a straight forward low pass ADC is used, it is necessary that the sampling frequency used to convert the analogue signal into a digital signal, is at least twice as great as the maximum frequency component contained in the analogue signal to be converted into digital. By providing a Very Low IF (VLIF) whereby the wanted signal is down-converted to be centred about an IF of approximately half the bandwidth of the wanted signal, the down-converted wanted signal will occupy a frequency band from approximately 0 Hz to the bandwidth of the wanted signal. This means that the sampling rate only needs to be about twice as large as the bandwidth of the wanted signal.
Preferably, the RF to IF down conversion stage outputs the IF signal as a complex IF signal comprising first and second Quadrature IF component signals. This is advantageous as it enables one to distinguish between signals and signal images located in the frequency range which is passed by the ADC (ie between minus the bandwidth of the wanted signal and plus the bandwidth of the wanted signal).
As mentioned above, it is preferred that the wanted signal, when down-converted to a VLIF signal, is centred about a VLIF which is about the same order of magnitude as the bandwidth of the wanted signal. In particular, it is preferred that the wanted signal, when down-converted to a VLIF signal, is centred about a VLIF which is about half the bandwidth of the wanted signal. The exact choice of VLIF about which the wanted signal is to be centred, will depend on exactly what type of signals the apparatus is to receive and process. In the case of GSM signals, it is preferred that the VLIF about which the wanted signal is centred, should be within the range of half the channel separation frequency xc2x110%. Ideally, it will be within the range of half the channel separation frequency +5%. However, in the case of signals having a greater order of modulation (as is anticipated for EDGE [Enhanced Data for GSM Evolution]) the VLIF about which the wanted signal should be centred, is preferably in the range of half the channel separation frequency +10%-20%. Note that the term channel separation frequency will be well understood by a person skilled in the art to mean the separation in frequency between adjacent channels defined by measuring corresponding points in the different channels (eg the distance between the midpoints of adjacent channels will equal the channel separation frequency).
Preferably, the complex notch filter includes first and second Finite Impulse Response (FIR) filters having different first and second sets of coefficients associated therewith respectively, wherein one of the sets of coefficients corresponds to the real parts of a set of complex coefficients and the other set of coefficients responds to the imaginary part of the same set of complex coefficients. By ensuring that both the I and Q components of the base-band signal are filtered by both the first and second FIR filters (thus generating four filtered signals) and combining the signals produced thereby in an appropriate manner, the result is that of an FIR filter having complex coefficients operating upon a complex signal whose imaginary and real parts are given by the Q and I component signals.
Preferably, the complex notch filter includes inversion means whereby the outputs of one or more of the FIR filters may be inverted to thereby alter the operation of the complex notch filter such that the complex notch filter substantially passes the small portion of the base-band signal centred about the first frequency while substantially filtering out the corresponding portion of the base-band signal centred about the second frequency.
Preferably, the apparatus is formed as an integrated circuit.
According to a second aspect of the present invention, there is provided apparatus for receiving and processing a wanted Radio Frequency signal comprising a radio frequency to intermediate frequency down-conversion stage for receiving the wanted radio frequency signal and outputting a complex intermediate frequency signal; an analogue to digital converter for converting the complex intermediate frequency signal to a digital complex intermediate signal; an intermediate frequency to base-band down-conversion stage for receiving the digital complex intermediate frequency signal and outputting a digital complex base-band signal; and a complex notch filter for receiving the digital complex base-band signal and outputting a notch filtered digital complex base-band signal wherein the complex notch filter substantially filters out a small portion of the base-band signal centred about a first, non-zero, frequency while substantially passing a corresponding portion of the base-band signal centred about a second frequency having the same magnitude but opposite sign to the first frequency.
According to a third aspect of the present invention, there is provided a method of receiving and processing a wanted Radio Frequency signal comprising the steps of receiving the wanted Radio Frequency signal and down-converting it to a complex intermediate frequency signal; converting the complex intermediate frequency signal from an analogue signal into a corresponding digital signal; converting the digital intermediate frequency to a digital base-band signal; and filtering the digital base-band signal with a complex notch filter in order to filter out a small portion of the base-band signal centred about a first, non-zero frequency whilst substantially passing a corresponding portion of the baseband signal centred about a second frequency having the same magnitude but opposite sign to the first frequency.